Reactions of Co(III)-nitrosyl complexes with superoxide and their mechanistic insights

Microbubble-encapsulated Cobalt Nitrato Complexes for Ultrasound-triggerable Nitric Oxide Delivery

Cobalt complexes exhibit versatile reactivity with nitric oxide (NO), enabling their utilization in applications ranging from homogeneous catalysis to NO-based modulation of biological processes. However, the coordination geometry around the cobalt center is complex, the therapeutic window of NO is narrow, and controlled NO delivery is difficult. To better understand the complexation of cobalt with NO, we prepared four cobalt nitrato complexes and present a structure-property relationship for ultrasound-triggerable NO release. We hypothesized that modulation of the coordination geometry by ligand-modification would improve responsiveness to mechanical stimuli, like ultrasound. To enable eventual therapeutic testing, we here first demonstrate the in vitro tolerability of [Co(ethylenediamine)2(NO)(NO3)](NO3) in A431 epidermoid carcinoma cells and J774A.1 murine macrophages, and we subsequently show successful encapsulation of the complex in poly(butyl cyanoacrylate) microbubbles. These hybrid Co-NO-containing microbubbles may in the future aid in ultrasound imaging-guided treatment of NO-responsive vascular pathologies.

Exploring the carbonic anhydrase-mimetic [(PMDTA)2ZnII2(OH-)2]2+ for nitric oxide monooxygenation

In biology, nitrite (NO2-) serves as a storage pool of nitric oxide (NO); however, the formation of NO2- from NO is still under investigation. Here, we report the NO monooxygenation (NOM) reaction of a ZnII-hydroxide complex (1), producing a ZnII-nitrito complex {2, (ZnII-NO2-)} + H2.

Mechanistic insights into nitric oxide oxygenation (NOO) reactions of {CrNO}5 and {CoNO}8

Here, we report the nitric oxide oxygenation (NOO) reactions of two distinct metal nitrosyls {Co-nitrosyl (S = 0) vs. Cr-nitrosyl (S = 1/2)}. In this regard, we synthesized and characterized [(BPMEN)Co(NO)]2+ ({CoNO}8, 1) to compare its NOO reaction with that of [(BPMEN)Cr(NO)(Cl-)]+ ({CrNO}5, 2), having a similar ligand framework. Kinetic measurements showed that {CrNO}5 is thermally more stable than {CoNO}8. Complexes 1 and 2, upon reaction with the superoxide anion (O2˙-), generate [(BPMEN)CoII(NO2-)2] (CoII-NO2-, 3) and [(BPMEN)CrIII(NO2-)Cl-]+ (CrIII-NO2-, 4), respectively, with O2 evolution. Furthermore, analysis of these NOO reactions and tracking of the N-atom using 15N-labeled NO (15NO) revealed that the N-atoms of 3 (CoII-15NO2-) and 4 (CrIII-15NO2-) derive from the nitrosyl (15NO) moieties of 1 and 2, respectively. This work represents a comparative study of oxidation reactions of {CoNO}8vs. {CrNO}5, showing different rates of the NOO reactions due to different thermal stability. To complete the NOM cycle, we reacted 3 and 4 with NO, and surprisingly, only 3 generated {CoNO}8 species, while 4 was unreactive towards NO. Furthermore, the phenol ring nitration test, performed using 2,4-di-tert-butylphenol (2,4-DTBP), suggested the presence of a proposed peroxynitrite (PN) intermediate in the NOO reactions of 1 and 2.

Reaction of a Co(III)-peroxo complex with nitric oxide: putative formation of a peroxynitrite intermediate

A Co(II) complex, [CoII(L)2(H2O)2](ClO4)2, 1, having a bidentate ligand L [L = bis(3,5-dimethylpyrazolyl)methane] has been synthesized. Complex 1 in acetonitrile solution at -40 °C, in the presence of H2O2 and NEt3, afforded the corresponding Co(III)-peroxo species, [CoIII(L)2(O22-)]+, as the transient intermediate 1a. Thermal instability precluded its isolation and further characterization. The addition of nitric oxide (NO) gas into the freshly prepared [CoIII(L)2(O22-)]+ in acetonitrile at -40 °C resulted in the corresponding Co(II)-nitrato complex, [CoII(L)2(NO3)](ClO4) (2). The reaction is proposed to proceed through a putative Co(II)-peroxynitrite intermediate 1b. It was evidenced by the characteristic phenol ring nitration reaction.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Nitric Oxide Oxygenation Reactions of Cobalt-Peroxo and Cobalt-Nitrosyl Complexes

Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of Co^III-peroxo (Co^III-O₂^2-) and Co-nitrosyl ({CoNO}⁸) complexes bearing the same N₄-donor ligand (HMTETA) framework. In this regard, we prepared and characterized two new [(HMTETA)Co^III(O₂^2-)]⁺ (2, S = 2) and [(HMTETA)Co(NO)]²⁺ (3, S = 1) complexes from [(HMTETA)Co^II(CH₃CN)₂]²⁺ (1). Both complexes (2 and 3) are characterized by different spectroscopic measurements, including their DFT-optimized structures. Complex 2 produces Co^II-nitrato [(HMTETA)Co^II(NO₃^-)]⁺ (Co^II-NO₃^-, 4) complex in the presence of NO. In contrast, when 3 reacted with a superoxide (O₂^•-) anion, it generated Co^II-nitrito [(HMTETA)Co^II(NO₂^-)]⁺ (Co^II-NO₂^-, 5) with O₂ evolution. Experiments performed using ^18/16O-labeled superoxide (¹⁸O₂^•-/¹⁶O₂^•-) showed that O₂ originated from the O₂^•- anion. Both the NOO reactions are believed to proceed via a presumed peroxynitrite (PN) intermediate. Although we did not get direct spectral evidence for the proposed PN species, the mechanistic investigation using 2,4-di-tert-butylphenol indirectly suggests the formation of a PN intermediate. Furthermore, tracking the source of the N-atom in the above NOO reactions using ¹⁵N-labeled nitrogen (¹⁵NO) revealed N-atoms in 4 (Co^II-¹⁵NO₃^-) and 5 (Co^II-¹⁵NO₂^-) derived from the ¹⁵NO moiety.

Acid-induced nitrite reduction of nonheme iron(ii)-nitrite: mimicking biological Fe-NiR reactions

Nitrite reductase (NiR) catalyzes nitrite (NO₂ ^-) to nitric oxide (NO) transformation in the presence of an acid (H⁺ ions/pH) and serves as a critical step in NO biosynthesis. In addition to the NiR enzyme, NO synthases (NOSs) participate in NO production. The chemistry involved in the catalytic reduction of NO₂ ^-, in the presence of H⁺, generates NO with a H₂O molecule utilizing two H⁺ + one electron from cytochromes and is believed to be affected by the pH. Here, to understand the effect of H⁺ ions on NO₂ ^- reduction, we report the acid-induced NO₂ ^- reduction chemistry of a nonheme Fe^II-nitrito complex, [(12TMC)Fe^II(NO₂ ^-)]⁺ (Fe^II-NO₂ ^-, 2), with variable amounts of H⁺. Fe^II-NO₂ ^- upon reaction with one-equiv. of acid (H⁺) generates [(12TMC)Fe(NO)]²⁺, {FeNO}⁷ (3) with H₂O₂ rather than H₂O. However, the amount of H₂O₂ decreases with increasing equivalents of H⁺ and entirely disappears when H⁺ reaches ≅ two-equiv. and shows H₂O formation. Furthermore, we have spectroscopically characterized and followed the formation of H₂O₂ (H⁺ = one-equiv.) and H₂O (H⁺ ≅ two-equiv.) and explained why bio-driven NiR reactions end with NO and H₂O. Mechanistic investigations, using ¹⁵N-labeled-¹⁵NO₂ ^- and ²H-labeled-CF₃SO₃D (D⁺ source), revealed that the N atom in the {Fe^14/15NO}⁷ is derived from the NO₂ ^- ligand and the H atom in H₂O or H₂O₂ is derived from the H⁺ source, respectively.

Why Intermolecular Nitric Oxide (NO) Transfer? Exploring the Factors and Mechanistic Aspects of NO Transfer Reaction

Small molecule activation & their transfer reactions in biological or catalytic reactions are greatly influenced by the metal-centers and the ligand frameworks. Here, we report the metal-directed nitric oxide (NO)...

Reaction of a {Co(NO)}8 complex with superoxide: Formation of a six coordinated [CoII(NO)(O2-)] species followed by peroxynitrite intermediate

A nitrosyl complex of cobalt(II) porphyrinate, [Co(F₂₀TPP^2-)(NO)], (F₂₀TPPH₂ = 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin) having {Co(NO)}⁸ configuration was synthesized and characterized by means of spectroscopic and structural analyses. Single crystal X-ray structure of the complex revealed the square pyramidal geometry around the cobalt center with a bent nitrosyl group. It reacts with superoxide (O₂^-) ion in CH₂Cl₂ at -40 °C to result in the corresponding nitrite (NO₂^-) complex. Involvement of a cobalt(II)-peroxynitrite intermediate is proposed in the course of the reaction. Moreover, spectroscopic studies suggested the formation of a transient six-coordinated [Co^II(NO)(O₂^-)] species.

Bent and Linear {CoNO}8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls

Among the isoelectronic ligands CN^-, CO, and NO⁺, an oblique bonding to the metal is well-established for the nitrosyl ligand, with M-N-O angles down to ≈120°. In the last decades, the nitrosyl community got into the habit of addressing a bent-bonded nitrosyl ligand as ¹NO^-. Thus, because various redox forms of a nitrosyl ligand seem to exist, the ligand is considered to be "noninnocent" because of the obvious ambiguity of an oxidation state (OS) assignment of the ligand and metal. Among the bent-bonded species, the low-spin {CoNO}⁸ class is prototypic. From this class, some 20 new nitrosyl compounds, the X-ray structure determinations of which comply with strict quality criteria, were analyzed with respect to the OS issue. As a result, the effective OS method shows a low-spin d⁸ Co^I-NO⁺ couple instead of a negative OS of the ligand at the BP86/def2-TZVP (+D3, +CPCM with infinite permittivity) level of theory. The same holds for some new members of the linear subclass of {CoNO}⁸ compounds. For all compounds, a largely invariable "real" charge of ≈ -0.3 e was obtained from population analyses. All of these electron-rich d⁸ species strive to manage Pauli repulsion between the metal electrons and the lone pair at the nitrosyl's nitrogen atom, with the bending of the CoNO unit as the most frequent escape.

Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO₃⁻) to nitrite (NO₂⁻) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO₂⁻ from NO₃⁻, which further produces nitric oxide (NO) either in acid-induced NO₂⁻ reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl₃ (V³⁺ ion source) induced step-wise reduction of NO₃⁻ in a Co^II-nitrato complex, [(12-TMC)Co^II(NO₃⁻)]⁺ (2,{Co^II–NO₃⁻}), to a Co^III–nitrosyl complex, [(12-TMC)Co^III(NO)]²⁺ (4,{CoNO}⁸), bearing an N-tetramethylated cyclam (TMC) ligand. The VCl₃ inspired reduction of NO₃⁻ to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, i.e., OAT-1 = NO₃⁻ → NO₂⁻ (r₁) and OAT-2 = NO₂⁻ → NO (r₂). In these OAT reactions, VCl₃ functions as an O-atom abstracting species, and the reaction of 2 with VCl₃ produces a Co^III-nitrosyl ({CoNO}⁸) with V^V-Oxo ({V^V Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O}³⁺) species, via a proposed Co^II-nitrito (3, {Co^II–NO₂⁻}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl₃, which showed the generation of 4 with V^V-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl₃ as a limiting reagent, as the second-order rate constant of OAT-2 (k₂^/) is found to be ∼1420 times faster than that of the OAT-1 (k₂) reaction. Binding constant (K_b) calculations also support our proposition of NO₃⁻ to NO transformation in two successive OAT reactions, as K_{b(Co^II–NO2⁻)} is higher than K_{b(Co^II–NO3⁻)}, hence the reaction moves in the forward direction (OAT-1). However, K_{b(Co^II–NO2⁻)} is comparable to K_b{CoNO}⁸, and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using ¹⁵N-labeled-¹⁵NO₃⁻ and ¹⁵NO₂⁻ revealed that the N-atom in the {CoNO}⁸ is derived from NO₃⁻ ligand. This work highlights the first-ever report of VCl₃ induced step-wise NO₃⁻ reduction (NRs activity) followed by the OAT induced NO₂⁻ reduction and then the generation of Co-nitrosyl species {CoNO}⁸.

Single metal-induced reduction of NO₃⁻ → {NO₂⁻} → NO via oxygen atom transfer reaction.

Nitric oxide monooxygenation (NOM) reaction of cobalt-nitrosyl {Co(NO)}8 to CoII-nitrito {CoII(NO2 -)}: base induced hydrogen gas (H2) evolution

Here, we report the nitric oxide monooxygenation (NOM) reactions of a CoIII-nitrosyl complex (1, {Co-NO}8) in the presence of mono-oxygen reactive species, i.e., a base (OH-, tetrabutylammonium hydroxide (TBAOH) or NaOH/15-crown-5), an oxide (O2- or Na2O/15-crown-5) and water (H2O). The reaction of 1 with OH- produces a CoII-nitrito complex {3, (CoII-NO2 -)} and hydrogen gas (H2), via the formation of a putative N-bound Co-nitrous acid intermediate (2, {Co-NOOH}+). The homolytic cleavage of the O-H bond of proposed [Co-NOOH]+ releases H2 via a presumed CoIII-H intermediate. In another reaction, 1 generates CoII-NO2 - when reacted with O2- via an expected CoI-nitro (4) intermediate. However, complex 1 is found to be unreactive towards H2O. Mechanistic investigations using 15N-labeled-15NO and 2H-labeled-NaO2H (NaOD) evidently revealed that the N-atom in CoII-NO2 - and the H-atom in H2 gas are derived from the nitrosyl ligand and OH- moiety, respectively.

Nitric oxide dioxygenation (NOD) reactions of CoIII-peroxo and NiIII-peroxo complexes: NODversusNO activation

A comparative study of “nitric oxide dioxygenationversusdioxygen or nitric oxide activation”.

Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Mn(II)-Porphyrinate in the Presence of Superoxide: Formation of a Mn(IV)-oxo Species through a Putative Peroxynitrite Intermediate

A nitrosyl complex of Mn^II-porphyrinate, [(F₂₀TPP)Mn^II(NO)], 1 (F₂₀TPPH₂ = 5,10,15,20 tetrakis(pentafluorophenyl)porphyrin), was synthesized and characterized. Spectroscopic and structural characterization revealed complex 1 as a penta-coordinated Mn^II-nitrosyl with a linear Mn-N-O (180.0°) moiety. Complex 1 does not react with O₂. However, it reacts with superoxide (O₂^-) in THF at -80 °C to result in the corresponding nitrate (NO₃^-) complex, 2, via the formation of a presumed Mn^III-peroxynitrite intermediate. ESI-mass spectrometry and UV-visible and X-band EPR spectroscopic studies suggest the generation of Mn^IV-oxo species in the reaction through homolytic cleavage of the O-O bond of the peroxynitrite ligand as proposed in NOD activity. The intermediate formation of the Mn^III-peroxynitrite was further supported by the well accepted phenol ring nitration which resembles the biologically well-established tyrosine nitration.

Finding a new pathway for acid-induced nitrite reduction reaction: formation of nitric oxide with hydrogen peroxide

Here, we report a new pathway for nitrite reduction chemistry, formation of cobalt-nitrosyl ({Co^II-NO}⁸) with H₂O₂ in the reaction of a Co^II-nitrito complex with a one-fold acid (H⁺) via the formation of a Co^II-nitrous acid intermediate ({Co^II-ONOH}). Mechanistic investigations using ¹⁵N-labeled-¹⁵NO₂^- revealed that the N-atom in the {Co^II-NO}⁸ complex is derived from the nitrito ligand, and H₂O₂ came from the homolysis of the ON-OH moiety. Spectral evidence supporting the formation of the Co^II-ONOH intermediate and the generation of H₂O₂ is also presented.

A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex-The Product of the Reaction of Nitrogen Monoxide (·NO(g)) with a Ferric-Superoxide Species

Peroxynitrite (-OON═O, PN) is a reactive nitrogen species (RNS) which can effect deleterious nitrative or oxidative (bio)chemistry. It may derive from reaction of superoxide anion (O2•-) with nitric oxide (·NO) and has been suggested to form an as-yet unobserved bound heme-iron-PN intermediate in the catalytic cycle of nitric oxide dioxygenase (NOD) enzymes, which facilitate a ·NO homeostatic process, i.e., its oxidation to the nitrate anion. Here, a discrete six-coordinate low-spin porphyrinate-FeIII complex [(PIm)FeIII(-OON═O)] (3) (PIm; a porphyrin moiety with a covalently tethered imidazole axial "base" donor ligand) has been identified and characterized by various spectroscopies (UV-vis, NMR, EPR, XAS, resonance Raman) and DFT calculations, following its formation at -80 °C by addition of ·NO(g) to the heme-superoxo species, [(PIm)FeIII(O2•-)] (2). DFT calculations confirm that 3 is a six-coordinate low-spin species with the PN ligand coordinated to iron via its terminal peroxidic anionic O atom with the overall geometry being in a cis-configuration. Complex 3 thermally transforms to its isomeric low-spin nitrato form [(PIm)FeIII(NO3-)] (4a). While previous (bio)chemical studies show that phenolic substrates undergo nitration in the presence of PN or PN-metal complexes, in the present system, addition of 2,4-di-tert-butylphenol (2,4DTBP) to complex 3 does not lead to nitrated phenol; the nitrate complex 4a still forms. DFT calculations reveal that the phenolic H atom approaches the terminal PN O atom (farthest from the metal center and ring core), effecting O-O cleavage, giving nitrogen dioxide (·NO2) plus a ferryl compound [(PIm)FeIV═O] (7); this rebounds to give [(PIm)FeIII(NO3-)] (4a).The generation and characterization of the long sought after ferriheme peroxynitrite complex has been accomplished.

A cobalt–nitrosyl complex with a hindered hydrotris(pyrazolyl)borate coligand: detailed electronic structure, and reactivity towards dioxygen

The cobalt–nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3⁻), and shows a linear Co–N–O unit.

Zinc hydridotriphenylborates supported by a neutral macrocyclic polyamine

A neutral macrocyclic polyamine 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me₄TACD) stabilizes cationic zinc hydridotriphenylborates that react with CO₂(1 atm) under insertion into the B–H bonds.

A Peroxynitrite Dicopper Complex: Formation via Cu-NO and Cu-O2 Intermediates and Reactivity via O-O Cleavage Chemistry

A mixed-valent Cu(I)Cu(II) complex, [Cu^I,II₂(UN-O^-)]²⁺ (1), reacts with NO_(g) at -80 °C to form [Cu^I,II₂(UN-O^-)(NO)]²⁺ (2), best described as a mixed-valent nitrosyl complex that has a ν(N-O) band at 1670 cm^-1 in its infrared (IR) spectrum. Complex 2 undertakes a one-electron oxidation via the addition of O_2(g) to generate a new intermediate, best described as a superoxide and nitrosyl adduct, [Cu^II₂(UN-O^-)(NO)(O₂^-)]²⁺ (3), based on its distinctively blue-shifted ν(N-O) band at 1853 cm^-1. Over the course of 20 min at -80 °C, 3 is converted to the peroxynitrite (PN) complex [Cu^II₂(UN-O^-)(^-OON═O)]²⁺ (4), which was characterized by low-temperature electrospray ionization mass spectrometry (ESI-MS) and IR spectroscopy; ν(N-O) absorptions at 1520 and 1640 cm^-1 have been assigned as cis- and trans-conformers of the PN ligand in 4. Alternatively, the superoxide complex [Cu^II₂(UN-O^-)(O₂^•-)]²⁺ (5) is found to react with NO_(g) to generate the same intermediate superoxide and nitrosyl adduct 3 (based on IR criteria), which likewise converts to the same PN complex 4. The O-O bond in 4 undergoes heterolysis in dichloromethane solvent and is postulated to produce nitronium ion, leading to ortho-nitration of 2,4-di-tert-butylphenol (DTBP). However, in 2-methyltetrahydrofuran as solvent, the O-O bond undergoes homolysis to generate ^•NO₂ (detected spectrophotometrically) and a putative higher-valent complex, [Cu^II,III₂(UN-O^-)(O^2-)]²⁺, that abstracts a H-atom from DTBP to give [Cu^II₂(UN-O^-)(OH)]²⁺ and a phenoxyl radical. The latter may dimerize to form the bis-phenol observed experimentally or couple with the ^•NO₂ present, leading to o-phenol nitration.

Factors That Control the Reactivity of Cobalt(III)-Nitrosyl Complexes in Nitric Oxide Transfer and Dioxygenation Reactions: A Combined Experimental and Theoretical Investigation

Metal-nitrosyl complexes are key intermediates involved in many biological and physiological processes of nitric oxide (NO) activation by metalloproteins. In this study, we report the reactivities of mononuclear cobalt(III)-nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(14-TMC)Co(III)(NO)](2+) and [(12-TMC)Co(III)(NO)](2+), in NO-transfer and dioxygenation reactions. The Co(III)-nitrosyl complex bearing 14-TMC ligand, [(14-TMC)Co(III)(NO)](2+), transfers the bound nitrosyl ligand to [(12-TMC)Co(II)](2+) via a dissociative pathway, {[(14-TMC)Co(III)(NO)](2+) → {(14-TMC)Co···NO}(2+)}, thus affording [(12-TMC)Co(III)(NO)](2+) and [(14-TMC)Co(II)](2+) as products. The dissociation of NO from the [(14-TMC)Co(III)(NO)](2+) complex prior to NO-transfer is supported experimentally and theoretically. In contrast, the reverse reaction, which is the NO-transfer from [(12-TMC)Co(III)(NO)](2+) to [(14-TMC)Co(II)](2+), does not occur. In addition to the NO-transfer reaction, dioxygenation of [(14-TMC)Co(III)(NO)](2+) by O2 produces [(14-TMC)Co(II)(NO3)](+), which possesses an O,O-chelated nitrato ligand and where, based on an experiment using (18)O-labeled O2, two of the three O-atoms in the [(14-TMC)Co(II)(NO3)](+) product derive from O2. The dioxygenation reaction is proposed to occur via a dissociative pathway, as proposed in the NO-transfer reaction, and via the formation of a Co(II)-peroxynitrite intermediate, based on the observation of phenol ring nitration. In contrast, [(12-TMC)Co(III)(NO)](2+) does not react with O2. Thus, the present results demonstrate unambiguously that the NO-transfer/dioxygenation reactivity of the cobalt(III)-nitrosyl complexes bearing TMC ligands is significantly influenced by the ring size of the TMC ligands and/or the spin state of the cobalt ion.

Superoxide Ion: Generation and Chemical Implications

Superoxide ion (O2(•-)) is of great significance as a radical species implicated in diverse chemical and biological systems. However, the chemistry knowledge of O2(•-) is rather scarce. In addition, numerous studies on O2(•-) were conducted within the latter half of the 20th century. Therefore, the current advancement in technology and instrumentation will certainly provide better insights into mechanisms and products of O2(•-) reactions and thus will result in new findings. This review emphasizes the state-of-the-art research on O2(•-) so as to enable researchers to venture into future research. It comprises the main characteristics of O2(•-) followed by generation methods. The reaction types of O2(•-) are reviewed, and its potential applications including the destruction of hazardous chemicals, synthesis of organic compounds, and many other applications are highlighted. The O2(•-) environmental chemistry is also discussed. The detection methods of O2(•-) are categorized and elaborated. Special attention is given to the feasibility of using ionic liquids as media for O2(•-), addressing the latest progress of generation and applications. The effect of electrodes on the O2(•-) electrochemical generation is reviewed. Finally, some remarks and future perspectives are concluded.

Effect of ligand denticity on the nitric oxide reactivity of cobalt(ii) complexes

NO reactivity of three Co(ii) complexes, 1, 2 and 3 have been studied in degassed methanol solution. The complexes differ from each other in terms of denticity and flexibility of the ligand fameworks. Complex 1 undergoes reductive nitrosylation of the metal ion; 2 results in corresponding [Co^III(NO⁻)] complex; whereas 3 does not react with NO.